Back in summer 1988 in Champaign, Illinois, I was a guest in a new house for dinner, and it used geothermal heat capacitance (or some similar term...basically i think water tubes buried a bit lower than the house) to essentially provide access to average year round temperature.

Since the climate there swings between extremes, this significantly lessened their electric/gas bills to the point of being laughably small...essentially a heat pump connected to the water tubes, if I remember.